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Tuning the isoelectric point of graphene by electrochemical functionalization.

Zuccaro L, Krieg J, Desideri A, Kern K, Balasubramanian K - Sci Rep (2015)

Bottom Line: The ability to control the charge-potential landscape at solid-liquid interfaces is pivotal to engineer novel devices for applications in sensing, catalysis and energy conversion.The isoelectric point (pI)/point of zero charge (pzc) of graphene plays a key role in a number of physico-chemical phenomena occurring at the graphene-liquid interface.The pI of bare graphene (as-prepared, chemical vapor deposition (CVD)-grown) is found to be less than 3.3, which we can continuously modify up to 7.5 by non-covalent electrochemical attachment of aromatic amino groups, preserving the favorable electronic properties of graphene throughout.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany.

ABSTRACT
The ability to control the charge-potential landscape at solid-liquid interfaces is pivotal to engineer novel devices for applications in sensing, catalysis and energy conversion. The isoelectric point (pI)/point of zero charge (pzc) of graphene plays a key role in a number of physico-chemical phenomena occurring at the graphene-liquid interface. Supported by theory, we present here a methodology to identify the pI/pzc of (functionalized) graphene, which also allows for estimating the nature and extent of ion adsorption. The pI of bare graphene (as-prepared, chemical vapor deposition (CVD)-grown) is found to be less than 3.3, which we can continuously modify up to 7.5 by non-covalent electrochemical attachment of aromatic amino groups, preserving the favorable electronic properties of graphene throughout. Modelling all the observed results with detailed theory, we also show that specific adsorption of ions and the substrate play only an ancillary role in our capability to tune the pI of graphene.

No MeSH data available.


Related in: MedlinePlus

The isoelectric point of bare (unmodified/as-prepared) graphene.(a) A 2D-map showing the evolution of gate dependence of graphene resistance (Rgr) as a function of varying pH and ionic strength (I − 1 mM, M − 1 M ionic strength; e.g. 6I refers to a pH 6 solution of 1 mM ionic strength). Every cycle takes around 10 s. The measurement is paused during solution exchange. VecG refers to electrochemical gate voltage applied through a Ag/AgCl reference electrode that is in contact with solution. The white profile superimposed on the 2D-map indicates the position of the Dirac point estimated from the profiles such as in (b). (b) Line profiles extracted from the 2D-map showing the gate dependence of graphene resistance in four different solutions, where the shift in Dirac point is discernible. (c) Measured and calculated Dirac point profiles as a function of pH at 2 different ionic strength values. (d) (red curves) measured and calculated difference Dirac curves obtained by subtracting the curve at 1 mM IS from that at 1 M IS (referred to as M − I). For comparison, the calculated surface charge density as a function of pH (green curve) is also superimposed. The difference curve is used as a measure to infer the sign of net surface charge on graphene. The simulated curves were obtained by assuming a pI of 2.0. See supplementary details for more information on model parameters.
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f2: The isoelectric point of bare (unmodified/as-prepared) graphene.(a) A 2D-map showing the evolution of gate dependence of graphene resistance (Rgr) as a function of varying pH and ionic strength (I − 1 mM, M − 1 M ionic strength; e.g. 6I refers to a pH 6 solution of 1 mM ionic strength). Every cycle takes around 10 s. The measurement is paused during solution exchange. VecG refers to electrochemical gate voltage applied through a Ag/AgCl reference electrode that is in contact with solution. The white profile superimposed on the 2D-map indicates the position of the Dirac point estimated from the profiles such as in (b). (b) Line profiles extracted from the 2D-map showing the gate dependence of graphene resistance in four different solutions, where the shift in Dirac point is discernible. (c) Measured and calculated Dirac point profiles as a function of pH at 2 different ionic strength values. (d) (red curves) measured and calculated difference Dirac curves obtained by subtracting the curve at 1 mM IS from that at 1 M IS (referred to as M − I). For comparison, the calculated surface charge density as a function of pH (green curve) is also superimposed. The difference curve is used as a measure to infer the sign of net surface charge on graphene. The simulated curves were obtained by assuming a pI of 2.0. See supplementary details for more information on model parameters.

Mentions: Graphene devices were fabricated by transferring CVD-graphene on to Si/SiO2 chips and patterning them using photolithography (See Methods for details)29. At the end of the fabrication process, we are left with a contacted graphene flake of size around 2 μm × 2.5 μm (see Fig. S5a), which is exclusively in contact with the solution with all lead electrodes passivated appropriately35. In order to ensure that the graphene surface is free of organic and trace metal impurities, we perform a rapid thermal annealing at 600 °C and an electrochemical etching procedure in HCl12. The field-effect in liquids is recorded by applying a gate voltage (through a Ag/AgCl reference electrode) and measuring the real part of impedance (Rgr) at a frequency of 1 kHz continuously in buffer solutions of varying pH and ionic strength (see supplementary information for preparation of buffer solutions)12. Figure 2(a) shows a typical measurement on bare graphene (as-prepared or unmodified graphene is referred to here as bare graphene) for varying pH values at two different ionic strengths (1 mM: I and 1000 mM: M). From every cycle of field-effect scan, as shown in Fig. 2(b), we extract the Dirac point ( or the gate voltage at resistance maximum), which is also overlaid in Fig. 2(a). The dotted lines in Fig. 2(c) show the dependence of this Dirac point as a function of pH for the two IS values, while that in Fig. 2(d) shows a plot of the M-I difference curve representative of the sign of surface charge on graphene.


Tuning the isoelectric point of graphene by electrochemical functionalization.

Zuccaro L, Krieg J, Desideri A, Kern K, Balasubramanian K - Sci Rep (2015)

The isoelectric point of bare (unmodified/as-prepared) graphene.(a) A 2D-map showing the evolution of gate dependence of graphene resistance (Rgr) as a function of varying pH and ionic strength (I − 1 mM, M − 1 M ionic strength; e.g. 6I refers to a pH 6 solution of 1 mM ionic strength). Every cycle takes around 10 s. The measurement is paused during solution exchange. VecG refers to electrochemical gate voltage applied through a Ag/AgCl reference electrode that is in contact with solution. The white profile superimposed on the 2D-map indicates the position of the Dirac point estimated from the profiles such as in (b). (b) Line profiles extracted from the 2D-map showing the gate dependence of graphene resistance in four different solutions, where the shift in Dirac point is discernible. (c) Measured and calculated Dirac point profiles as a function of pH at 2 different ionic strength values. (d) (red curves) measured and calculated difference Dirac curves obtained by subtracting the curve at 1 mM IS from that at 1 M IS (referred to as M − I). For comparison, the calculated surface charge density as a function of pH (green curve) is also superimposed. The difference curve is used as a measure to infer the sign of net surface charge on graphene. The simulated curves were obtained by assuming a pI of 2.0. See supplementary details for more information on model parameters.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4488746&req=5

f2: The isoelectric point of bare (unmodified/as-prepared) graphene.(a) A 2D-map showing the evolution of gate dependence of graphene resistance (Rgr) as a function of varying pH and ionic strength (I − 1 mM, M − 1 M ionic strength; e.g. 6I refers to a pH 6 solution of 1 mM ionic strength). Every cycle takes around 10 s. The measurement is paused during solution exchange. VecG refers to electrochemical gate voltage applied through a Ag/AgCl reference electrode that is in contact with solution. The white profile superimposed on the 2D-map indicates the position of the Dirac point estimated from the profiles such as in (b). (b) Line profiles extracted from the 2D-map showing the gate dependence of graphene resistance in four different solutions, where the shift in Dirac point is discernible. (c) Measured and calculated Dirac point profiles as a function of pH at 2 different ionic strength values. (d) (red curves) measured and calculated difference Dirac curves obtained by subtracting the curve at 1 mM IS from that at 1 M IS (referred to as M − I). For comparison, the calculated surface charge density as a function of pH (green curve) is also superimposed. The difference curve is used as a measure to infer the sign of net surface charge on graphene. The simulated curves were obtained by assuming a pI of 2.0. See supplementary details for more information on model parameters.
Mentions: Graphene devices were fabricated by transferring CVD-graphene on to Si/SiO2 chips and patterning them using photolithography (See Methods for details)29. At the end of the fabrication process, we are left with a contacted graphene flake of size around 2 μm × 2.5 μm (see Fig. S5a), which is exclusively in contact with the solution with all lead electrodes passivated appropriately35. In order to ensure that the graphene surface is free of organic and trace metal impurities, we perform a rapid thermal annealing at 600 °C and an electrochemical etching procedure in HCl12. The field-effect in liquids is recorded by applying a gate voltage (through a Ag/AgCl reference electrode) and measuring the real part of impedance (Rgr) at a frequency of 1 kHz continuously in buffer solutions of varying pH and ionic strength (see supplementary information for preparation of buffer solutions)12. Figure 2(a) shows a typical measurement on bare graphene (as-prepared or unmodified graphene is referred to here as bare graphene) for varying pH values at two different ionic strengths (1 mM: I and 1000 mM: M). From every cycle of field-effect scan, as shown in Fig. 2(b), we extract the Dirac point ( or the gate voltage at resistance maximum), which is also overlaid in Fig. 2(a). The dotted lines in Fig. 2(c) show the dependence of this Dirac point as a function of pH for the two IS values, while that in Fig. 2(d) shows a plot of the M-I difference curve representative of the sign of surface charge on graphene.

Bottom Line: The ability to control the charge-potential landscape at solid-liquid interfaces is pivotal to engineer novel devices for applications in sensing, catalysis and energy conversion.The isoelectric point (pI)/point of zero charge (pzc) of graphene plays a key role in a number of physico-chemical phenomena occurring at the graphene-liquid interface.The pI of bare graphene (as-prepared, chemical vapor deposition (CVD)-grown) is found to be less than 3.3, which we can continuously modify up to 7.5 by non-covalent electrochemical attachment of aromatic amino groups, preserving the favorable electronic properties of graphene throughout.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany.

ABSTRACT
The ability to control the charge-potential landscape at solid-liquid interfaces is pivotal to engineer novel devices for applications in sensing, catalysis and energy conversion. The isoelectric point (pI)/point of zero charge (pzc) of graphene plays a key role in a number of physico-chemical phenomena occurring at the graphene-liquid interface. Supported by theory, we present here a methodology to identify the pI/pzc of (functionalized) graphene, which also allows for estimating the nature and extent of ion adsorption. The pI of bare graphene (as-prepared, chemical vapor deposition (CVD)-grown) is found to be less than 3.3, which we can continuously modify up to 7.5 by non-covalent electrochemical attachment of aromatic amino groups, preserving the favorable electronic properties of graphene throughout. Modelling all the observed results with detailed theory, we also show that specific adsorption of ions and the substrate play only an ancillary role in our capability to tune the pI of graphene.

No MeSH data available.


Related in: MedlinePlus